Tripotassium phosphate treatment for strengthening glass

ABSTRACT

Strengthening glass articles by spraying the same with a water solution of tripotassium phosphate (K3 PO4) at a temperature slightly below the strain point of the glass, then holding the glass at such temperature for a time period sufficient to provide exchange of potassium ions for sodium ions of the glass surface, the potassium ions being of larger atomic diameter than the sodium ions whereby a surface compressive stress is created.

United States Patent [72] lnventors James P. Poole;

Herbert C. Snyder; Mark A. Boschini, all of Brockway, Pa.

[21] Appl. No. 772,835

[22] Filed Nov. 1, 1968 [45] Patented Sept. 21, 1971 [73] AssigneeBrockway Glass Company, Inc.

Brockway, Pa.

[54] TRIPOTASSIUM PHOSPHATE TREATMENT FOR STRENGTHENING GLASS 6 Claims,No Drawings [52] US. Cl. 65/30, 65/60,65/114, 117/124 [51] Int. Cl.C031: 21/00 [50] Field of Search 65/30, 60,

Primary Examiner-S. Leon Bashore Assistant Examiner-John H. HarmanAttorney-Christel & Bean ABSTRACT: Strengthening glass articles byspraying the same with a water solution of tripotassium phosphate (K POat a temperature slightly below the strain point of the glass, thenholding the glass at such temperature for a time period sufficient toprovide exchange of potassium ions for sodium ions of the glass surface,the potassium ions being of larger atomic diameter than the sodium ionswhereby a surface compressive stress is created.

TRIPOTASSIUM PHOSPHATE TREATIVENT FOR STRENGTHENING GLASS BACKGROUND OFTHE INVENTION This invention relates to a method of increasing themechanical strength of glass articles by chemical tempering.

Present methods of tempering glass articles to increase the breakingstrength consist in providing a surface layer of glass which is incompression. Since glass always fails in tension and fractures generallyoriginate at the surface of the glass, tempering the glass byestablishing a compressive stress in the surface layer strengthens theglass in accordance with the degree of this compressive stress. This isdue to the fact that a tensile stress will not be established at thesurface layer until a sufficient strain is applied to first overcome thecompressive stress.

The best known method of strengthening glass articles by establishing asurface compressive stress is by thermal tempering wherein the glass iscooled at a controlled rate, relatively rapidly, down through its strainpoint. In such cooling the surface layer passes below the strain pointof the glass and thus solidifies before the interior thereof so that thesubsequent cooling of the interior of the glass with its consequentshrinking establishes a compressive stress in the outer surface of theglass which is in equilibrium with a corresponding tensile stress in theinterior of the glass.

Various other methods of establishing a compressive layer at the surfaceof a glass article have been proposed, one such method being shown andclaimed in Weber U.S. Pat. No. 3,218,220 dated Nov. 16, 1965. Accordingto this patent potassium ions are substituted for sodium ions at thesurface layer of the glass and since the potassium ions are of largeratomic diameter the surface layer is thus placed under compression.

According to the Weber patent the exchange of potassium ions for sodiumions is accomplished by immersing the glass article in a moltenpotassium salt bath. This method is effective but is relativelyexpensive and time consuming and is consequently not readily applicableto glass articles of low cost and/or articles which must be producedrapidly on a massproduction basis.

SUMMARY OF THE INVENTION The present invention provides a method wherebypotassium ions may be exchanged for sodium ions in the surface layer ofa glass article rapidly and as an incident in the mass production ofsuch articles. Speaking generally, this is accomplished by spraying anaqueous tripotassium phosphate solution on the glass surface while theglass is at a temperature of about 950 F. and then soaking" the glass byholding the same at approximately the same temperature for aboutone-half hour to permit the exchange of potassium ions for sodium ionsto take place to a sufficient degree to establish a compressive stressby reason of the larger atomic diameter of the replacing potassium ions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS While several potassiumcompounds, and more particularly several potassium phosphate compounds,have proved useful in effecting the desired potassium-for-sodium ionexchange, we have found that unusual results are achieved by the use oftripotassium phosphate (K P0,).

Application of the K PO, is by spraying an aqueous solution (preferablyabout 50 percent) of the K F0, on glass articles while they are in aheated condition so that the water flashes off, leaving the potassiumsalt deposited on the hot glass surface.

In the following examples the glass was held at the approximatetemperature of application of the tripotassium phosphate solution forapproximately one-half hour to permit the exchange of potassium ions forsodium ions to be effected sufficiently to establish the desiredcompressive stress layer.

In the following examples tests were made on rods having an averagediameter of 0.110 inch (chosen because this approximates the bottlesidewall thickness) and the composition of the glass tested wasapproximately as follows:

so, 7 l .5 Fe, 0, 0.042 A1,0, 1.25 CaO 10.25

530 0.035 Na,0 |s.s K,O 0.023 so, 0.26 "no, 0.014

The foregoing is a conventional soda-lime-silica container glasscomposition and has a softening point of l,327 R, an annealing point ofl,022 F. and a strain point of 986 F.

The rods were tested in groups of 25 to 30 rods each and the rods werebroken under uniform conditions and the breaking strengths weretabulated.

A number of groups of rods were treated by heating the rods to 950 F.,spraying a 50 percent solution of K P0, or K, I'IPO, as will beexplained below, and were then returned to the furnace and held at the950 F. temperature for one-half hour, then cooled to room temperaturefor testing.

To serve as a basis of comparison, seven groups of 25 to 30 rods each ofthe above composition were annealed, being otherwise untreated. Thelowest average strength of the rods of the untreated group was 14,200p.s.i. and the highest 22,100 p.s.i. The average breaking strength ofthe several groups was slightly below 18,000 p.s.i.

Seventeen groups of 25 to 30 rods each were treated in the foregoingmanner with aqueous solutions of K PO, and upon testing the group havingthe lowest average breaking strength averaged 39,000 p.s.i. and thegroup having the highest average breaking strength averaged 77,000p.s.i. The overall average of the 17 groups of rods thus treated withK;, 3 4 and tested was 54,000 p.s.i.

Similar tests on like rods under the same condition performed on fivegroups of rods but employing dipotassium hydrogen phosphate (K HPOproduced the following average breaking strengths. The lowest averagebreaking strength of these five groups of 25 to 30 rods each was 24,000p.s.i. and the highest was 43,000 p.s.i., the average of the five groupsbeing 37,000 p.s.i.

From this it will be seen that the average breaking strengths of theforegoing tests are as follows:

Untreated rods-18,000 p.s.i.

Rods treated with K Fo -54,000+ p.s.i.

Rods treated with K HPO 37,000 p.s.i.

From the foregoing it is clear that the average breaking strength of therods treated with K P0, was three times that of the untreated rods andapproximately 50 percent greater than that of the rods treated with KHPO The following test was made on nonretumable beer bottles of the samecomposition as specified above. The bottles were sprayed with a 25percent K P0 solution at between 500 and 600 F. and run through anannealing lehr. ln the lehr sprayed bottles were raised to a temperatureof approximately l,000 F. and held at that temperature for about 5minutes and then cooled in about 15 minutes. This produced a skincompression of about 3,000 p.s.i. approximately 30 microns deep.

The average hydrostatic bursting strength of untreated annealed bottleslike those of the foregoing test and of the same glass composition is435 p.s.i. and the average hydrostatic bursting strength of 20 bottlestreated as specified in the preceding paragraph was 629 p.s.i.

While this strength increase ratio is substantially less than thatexperienced in the rod treatments detailed above, the conditions ofspray application and the limited times for which the bottles were heldat elevated temperature would account in part for lower increase in thestrength of the glass.

Furthermore, in spraying the bottles in the foregoing bottle test, sprayequipment and methods were of a rather crude nature so that uniform andcomplete spray application could not be expected. Also, these tests weremade with a 25 percent K PO solution and the temperature of applicationwas about one-third lower than the temperature of application in thecase of the above glass rod treatments. Another factor affecting thebottle test results is the complex geometry of glass bottles whichproduces unpredictable stress concentrations. All of these factors andothers account for the lower strength increase in the case of the bottletests although the noted increase in strength is still very substantial.

The strength of the tripotassium phosphate solution is variable throughwide limits since solutions of lower strengths can be employed byspraying more solution on the glass. This is limited by practicalconsiderations and also by the danger of thermal shock if too much watersolution is sprayed on the glass. In the glass rod tests solutions of 50percent K P were employed and in the above bottle tests 25 percentsolutions were employed. This solution strength may be as low as percentor percent, in which case more solution would be sprayed on the bottle.

After the foregoing treatments the glass articles are washed withordinary tap water to remove excess salt from the glass surface andleave the same clear and without haze. The salt thus washed from thesurface may be reused within limits although reuse cannot be carried onafter the sodium salt content of the reclaimed material becomes too highrelative to the potassium salt content.

We claim:

l. A method of strengthening a soda-lime glass body by substitutingpotassium ions for sodium ions in a surface layer of the glass toproduce a compressive surface layer, said method comprising sprayingtripotassium phosphate (K PO.) on the surface of the glass body withsaid surface at a temperature from about 500 F. to about the annealing.point of the glass and maintaining the glass at a temperature from 500F. to l,050 F. for approximately 20 minutes, but in any case for a timeinsufficient to produce stress relaxation, to provide a substantialexchange of potassium ions for sodium ions to a depth sufficient toproduce a substantial compressive surface layer, cooling the glass body,and removing the residual surface salt to produce a clear, strengthenedglass body.

2. The method of claim 1 wherein the tripotassium phosphate is depositedagainst the heated glass body in water solution, the temperature of theglass bodies being sufficient to rapidly evaporate the solution.

3. The method of claim 2 wherein the water solution contains at least 10percent of tripotassium phosphate.

4. The method of claim 1 wherein the glass body is maintained at atemperature between 500 F. and l,050 F. for approximately 20 minutes orlonger.

5. The method of claim 2 wherein the aqueous tripotassium phosphatesolution is sprayed on a heated glass body immediately after formationthereof.

6. The method of claim 5 wherein the elevated temperature for ionexchange reaction is provided in the annealing treatment of the glassbody.

2. The method of claim 1 wherein the tripotassium phosphate is depositedagainst the heated glass body in water solution, the temperature of theglass bodies being sufficient to rapidly evaporate the solution.
 3. Themethod of claim 2 wherein the water solution contains at least 10percent of tripotassium phosphate.
 4. The method of claim 1 wherein theglass body is maintained at a temperature between 500* F. and 1,050* F.for approximately 20 minutes or longer.
 5. The method of claim 2 whereinthe aqueous tripotassium phosphate solution is sprayed on a heated glassbody immediately after formation thereof.
 6. The method of claim 5wherein the elevated temperature for ion exchange reaction is providedin the annealing treatment of the glass body.